The causes of bruising on the arm for no reason are considered to be abnormal coagulation function of the organism and dysfunction of the immune system, as follows: 1. Abnormal coagulation function of the organism: mainly including the lack of certain coagulation factors in the coagulation factors of the organism, or due to dysfunction of the immune system of the organism, resulting in hypersplenism, causing excessive platelet destruction, resulting in a low number of platelets, which can cause coagulation function of the organism The abnormal blood clotting function can lead to bruises on the arms for no reason. In addition, if you take aspirin and other blood-activating, anti-inflammatory and analgesic drugs for a long time, it may also affect the coagulation system of the body, which may lead to bruising; 2. The dysfunction of the immune system of the body: this will cause increased vascular permeability and extravasation of blood, causing local bruising. To treat bruising, first of all, you need to go to the hematology department of the hospital for routine blood and coagulation system examination to clarify, and at the same time, you should pay attention to avoid trauma and other situations to avoid aggravating bruising.